1. Field of the Invention
The present invention relates to an electric-component mounting system arranged to mount electric components (including electronic components) on a circuit substrate such as a printed-wiring board, and to a method of obtaining relative positions of specific sections of the electric-component mounting system that influence the accuracy of mounting of the electric components.
2. Discussion of Related Art
Electric-component mounting systems often use a suction nozzle arranged to hold an electric component by suction under a negative pressure. Usually, the actual position of the electric component held by the suction nozzle deviates from the nominal position. In view of this positional deviation of the electric component, it has been practiced to detect an error of positioning of the electric component with respect to the suction nozzle by operating an image-taking device to take an image of the electric component as held by the suction nozzle, and compensate the position of the electric component for the detected positioning error before the electric component is mounted on the circuit substrate. The positioning error includes at least one of an error of positioning of the center position or other reference position of the electric component in a plane perpendicular to an axis of rotation of the suction nozzle, and an error of angular positioning of the electric component about the axis of rotation of the suction nozzle. The circuit substrate on which the electric components are mounted is usually positioned by a substrate supporting device. However, the actual positions of pads in a circuit pattern formed on the circuit substrate, on which the electric components are to be mounted, more or less deviate from the nominal positions. In view of this deviation, it is practiced to form fiducial marks on the circuit substrate upon formation of the circuit pattern, detect the positioning error of the circuit substrate by operating an image-taking device to take images of the fiducial marks, estimate the positional deviation of the pads on the basis of the detected positioning error, and compensate the position of each electric component for the estimated positional deviation before the electric component is mounted on the corresponding pad.
To obtain the positioning error of the electric component with respect to the suction nozzle and the positioning error of the fiducial marks on the circuit substrate with respect to the substrate supporting device, it is necessary to know the positions of the electric component and the fiducial marks relative to the suction nozzle and the image-taking devices when the images of the electric component and the fiducial marks are taken by the image-taking devices. The positioning errors of the electric component and the fiducial marks may be obtained by detecting those relative positions using exclusively designed detecting devices, when the electric-component mounting system is assembled or inspected for maintenance purposes. In this case, the obtained positioning errors are used on an assumption that the detected relative positions are maintained in operation of the system. To obtain the positioning errors in this manner, the detecting devices are required to be complicated and tend to be expensive. Further, ballscrews used in relative-movement devices provided to move the suction nozzle, substrate supporting device and image-taking devices relative to each other undergo elongation and contraction due to a change in the temperature, and elastic deformation due to loads acting thereon. In addition, the machine frame of the electric-component mounting system, brackets which hold the image-taking devices, and other portions of the system are also subject to deformation due to a temperature variation. Accordingly, the actual relative positions of the suction nozzle, substrate supporting device and image-taking devices may vary with respect to the detected relative positions, due to the elongation and contraction and elastic deformation of the ballscrews of the relative-movement devices, and deformation of the machine frame and brackets. The above-indicated elongation and contraction and deformation due to the temperature variation will be collectively referred to as “thermal deformation”.
It is further noted that the relative positions of the suction nozzle, substrate supporting device and image-taking devices may vary due to chronological changes of the individual elements of the electric-component mounting system, wearing of the elements during use, and displacements of the elements due to loosening of fastening or fixing devices used in the system. To reduce this variation of the relative positions in an electric-component mounting system which is required to assure a high degree of component mounting accuracy, it has been practiced to provide the system with linear encoders to detect the actual relative positions of the suction nozzle, substrate supporting device and image-taking devices, and/or form the brackets for supporting the image-taking devices, of highly rigid materials having a relatively low coefficient of thermal expansion, and/or design the lenses of the image-taking devices such that the lenses are highly resistant to vibrations. However, such measures inevitably result in an increase in the cost of manufacture of the electric-component mounting system, and cannot therefore be said to be completely satisfactory.